Restructuring Specifications: Improving Stability and Durability of Structural Concrete With ACI 301-16
What can be done to help specifiers and contractors create the most stable, durable, and resilient concrete structures possible? This challenge inspired the industry professionals responsible for updating an American Concrete Institute (ACI) standard over the summer. (ACI 301-16 is available in print and digital formats at www.concrete.org.)
Every five years, ACI 301, Specifications for Structural Concrete, is updated to complement the newest version of ACI 318, Building Code Requirements for Structural Concrete. (ACI 318-14, Building Code Requirements for Structural Concrete, has been completely reorganized to reflect the designer’s perspective. More details are available at www.concrete.org/tools/ACI318.) During the most recent review cycle, ACI 301 Committee on Specifications for Structural Concrete not only updated technical requirements, but also created a more user-friendly reference specification for nearly all forms of structural concrete.
The committee’s approach was twofold. Specific wording was one area of focus: removing redundancies, making language more self-explanatory, and improving definitions of terms. The specification also had to maintain a broad scope, since ACI 301-16 is used across a wide range of climates, geographic areas, and social conditions around the world.
One way of encouraging specifiers and contractors to adopt a specification is to make it easier to understand.
“We spent a lot of time cleaning up the language to make it more clear for the user,” says Michelle Wilson, the committee’s chair (and director of concrete technology for the Portland Cement Association [PCA]). “This is a contract document written in mandatory language, so it’s very important to remove wording that is confusing or subject to interpretation.”
The committee clarified many commonly used terms. ‘Architectural concrete,’ for example, is defined in the specification as:
concrete that is typically exposed to view, is designated as architectural concrete in contract documents, and therefore requires care in selection of the concrete materials, forming, placing, and finishing to obtain the desired architectural appearance.
Many provisions have been spelled out to clearly indicate their intent. For example, the requirement for water used for curing concrete previously stated:
Unless otherwise specified, water complying with the requirements for ASTM C1602/C1602M [Standard Specification for Mixing Water Used in the Production of Hydraulic Cement Concrete] is acceptable as curing water.
ACI 301-16 now reads:
Unless otherwise specified, do not use seawater or water containing substances that will discolor or impair the durability of a concrete member.
Throughout the specification, consistent language differentiates between required and optional provisions. The phrase “as specified in the contract documents” indicates a mandatory checklist item and tells specifiers they must take action. “Unless specified otherwise” means something is optional. This flags items specifiers may want to review, and tells contractors alternatives may be available. This ‘trigger language’ has been reduced from around 30 different words and phrases to just five or six in the new specification.
Given ACI 301 is used around the world, it is available in metric units, and a Spanish version will be published this year. However, ensuring the specification is truly applicable anywhere from Peru to Canada takes more than simple translation.
“ACI 301 is a conservative specification that covers many different applications. There are some restrictive provisions that are intended to provide structural, stable, and durable concrete in service,” says Wilson. “However, there are several contractor-friendly options that ensure we’re not restricting the builder’s ability to decide means and methods.”
In fact, many changes to ACI 301-16 are performance-based to better reflect standard industry practices. Concrete slump, for example, is no longer defined by the specifier. Contractors must choose, to a specific number of inches, the slump best-suited for their given application, then place concrete within the specified slump tolerance.
Previous versions of ACI 301 also included a checklist of submittal items, which was cumbersome for contractors and specifiers to utilize. Submittals are now defined in the body of the specification, so one does not need to jump between the body and a checklist at the back of the document.
DRAWING ON INDUSTRY EXPERTISE
As a specification rooted in actual jobsite practice, ACI 301-16 reflects the complexity of concrete construction—sometimes there is no straightforward answer.
“We took on several controversial subjects and weren’t always able to reach consensus,” says Jim Cornell, general superintendent at Beck Group (Fort Worth, Texas), and past chair of the ACI 301 Committee.
Cornell says, for example, descriptions of what may qualify as mass concrete in ACI 301-10 were “bulky, non-specific, and open for interpretation.”
After a concerted effort to refine the standard, the committee was unable to reach a better solution. In the end, the cementitious materials content—300 kg/m3 (660 lb/cy)—which had led to much debate, was simply removed.
New to ACI 301-16 are specification provisions addressing alkali-silica reactivity (ASR). ACI Committee 301 expended considerable effort on writing requirements providing for the long-term durability of structures, using the best, most current knowledge of test methods and mechanisms of failure. The committee focused on developing requirements usable across many geographic locations, despite the set of challenges each region presents to providing ASR-resistant materials for concrete. The published specifications provide clear direction for the testing of aggregates, as well as a choice of performance and prescriptive approaches for reducing the risk of ASR.
In other instances, committee members referred to specifications generated by industry associations outside of ACI.New to ACI 301-16 are specification provisions addressing alkali-silica reactivity (ASR). ACI Committee 301 expended considerable effort on writing requirements providing for the long-term durability of structures, using the best, most current knowledge of test methods and mechanisms of failure. The committee focused on developing requirements usable across many geographic locations, despite the set of challenges each region presents to providing ASR-resistant materials for concrete. The published specifications provide clear direction for the testing of aggregates, as well as a choice of performance and prescriptive approaches for reducing the risk of ASR.
“Certain topics may not have been covered in-depth by ACI 301 or other ACI specifications before, but other industry experts have developed comprehensive mandatory-language reference documents,” says Cornell.
ACI 301-16 now includes external references for topics such as reinforcement and post-tensioned concrete.
The specification refers contractors to a new document from the Concrete Reinforcing Steel Institute (CRSI) to determine proper support for welded wire reinforcement (CRSI RB4.1 , Supports for Reinforcement Used in Concrete, covers the design, use, and material requirements of various reinforcement supports used in concrete. For more information, visit www.crsi.org.), as well as a specification published by the Post Tensioning Institute (PTI) for grouting of bonded tendons. (PTI M55.1-12, Specification for Grouting of Post-Tensioned Structures, provides minimum requirements for the selection, design, and installation of cementitious grouts and ducts for post-tensioning systems used in concrete construction. Additional information can be found at www.post-tensioning.org.)
MANDATORY VERSUS OPTIONAL PROVISIONS
ACI 301-16 is organized into two parts, distinguishing basic items always required for structural concrete construction from those that may be required on particular projects. When an architect or engineer cites ACI 301-16, the document is intended to be adopted in its entirety into the contract documents. However, specifiers have the latitude to change some of its provisions.
The first five sections cover general construction requirements for contractors performing cast-in-place structural concrete work and placing slabs on-ground. These include requirements relating to formwork, reinforcement, concrete materials, and means and methods for placement and transportation.
The remaining nine sections are devoted to other structural concrete applications, which may or may not pertain to specific projects:
- architectural concrete;
- post-tensioned concrete;
- mass concrete;
- lightweight concrete;
- industrial floor slabs;
- shrinkage-compensating concrete;
- tilt-up construction; and
- both architectural and structural precast concrete.
These specialty provisions have been expanded in ACI 301-10, so each is discussed in a similar level of detail. Specifiers are required to indicate where any of these applications are used in a project.
At the end of the document, specifiers will find two checklists spelling out which provisions within each section are mandatory and which are optional.
“As a designer prepares contract documents referencing ACI 301, he or she can run through the checklists to ensure the right provisions are incorporated into the project,” according to Cornell.
The list of optional provisions serves as both a reference and a reminder.
“When designers review the optional checklist items, they may consider enhancements to the project that they hadn’t thought of before,” says Cornell. “It also lets contractors see potential avenues of accomplishing the work differently, and changes or improvements that could be made via substitution or a substitution request.”
With clarified language and structure, ACI’s newly updated standard, Specifications for Structural Concrete, provides a straightforward, contractor-accessible reference architects and engineers can apply to any project involving structural concrete. In addition to being cited in project specifications, ACI 301-16 requires contractors to keep a copy of the ACI Field Reference Manual—which includes the most recent structural concrete specifications—in the field office of any project where ACI 301 is specified. (The ACI Field Reference Manual is a compilation of documents incorporating ACI 301-16 and additional ACI documents on measuring, mixing, transporting, and placing concrete, concrete pumping methods, hot- and cold-weather concreting, consolidation, and concrete formwork.)
This year, ACI will also be publishing updated specifications designed to improve concrete testing and inspection services.
- ACI 311.6, Specification for Ready Mixed Concrete Testing Services; and
- ACI 311.7, Inspection Services Specification for Cast-in-place Concrete Construction.
Shelby O. Mitchell is a freelance writer for the construction industry, and former editor of The Concrete Producer magazine. She is based in Berwyn, Illinois. Mitchell can be reached at
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